1. Field of the Invention
The present invention relates to toner for electrostatic latent image development used for copying machines, printers and the like, a method for manufacture thereof, as well as a developer, an image forming method and an image forming apparatus using the same.
2. Description of the Related Art
Recently, in methods for forming images by electrophotography, along with progress of digital technology, an image formation in a digital mode has become predominant. In the method for forming images in the digital mode, it is a basic to manifest a dot image with a small pixel size such as 1200 dpi (dpi: dot number per inch, i.e., 2.54 cm). High-quality picture technology which faithfully reproduces these small dot images has been required, and in terms of such high-quality picture technology, it has been carried forward to make particle sizes of toner small.
As ways for achieving to make particle sizes of toner small and make a particle size distribution and shapes uniform, polymerization toners obtained by suspension polymerization and emulsion polymerization have been noticed. In Tokugan-Hei 11-304004, it is described that the shape, roundness and the particle size distribution can be controlled in the toner obtained by the polymerization.
As methods for fixing a toner picture formed on an image support (sometimes referred to as a transfer material or transfer paper) such as paper, a heat roller fixing mode is widely employed where the image support on which the toner picture has been formed is fixed by passing between a heating roller and a pressing roller. The heat roller fixing mode has a drawback that an offset phenomenon where the melted toner adheres to the heating roller occurs to easily cause image stain.
In order to prevent the offset phenomenon from occurring, it has been known to apply silicone oil on the heating roller of a fixing unit to impart mold release property for the toner to the heating roller. This method has been advantageous in terms of unlimited toner types, but unsuitable for business documents because writing with pens and pencils such as a ballpoint pen becomes difficult due to adhesion of silicone oil onto the transfer paper. In the market, copied documents are instantly used as business documents and writing onto the document is routinely performed. Thus, the method for applying the silicone oil has not met the needs of the times.
In these years, a technology where the mold release property is imparted to the toner itself by adding a mold release agent such as wax in the toner has been employed. In JP-Hei 3-296067A, those where a structure in which polypropylene, a mold release agent is dispersed in a binding resin is formed at a cross section of toner, and a maximum diameter in a long axis direction at island parts of the polypropylene and a mean distance between the islands are specified are disclosed. In JP-Hei 10-161338A, with respect to a mold release agent component of toner in a sea-island structure, those where particle sizes of the mold release agent component before heat fixing are specified and the sea-island structure disappears after the heat fixing are disclosed.
However, even if the mold release agent is introduced in the toner in this way, it has been still necessary to install a cleaning mechanism of a fixing roller. Because although in the toner where the mold release agent is introduced, the offset does not occur in the image formation where printing of several units is performed, when ten to twenty thousand sheets of printing are carried out, then, the toner is gradually accumulated onto the fixing roller and the offset so-called termed non-visual offset occurs. Also in terms of prevention of occurrence of non-visual offset, it is necessary to exchange a cleaning member of the fixing unit (fixing device) every processing of several ten thousand sheets.
Processing performance of the image forming apparatus has been improved and high speed machines having output of 50 or more sheets per minute have appeared. But, in such high speed machines, the fixing is completed before the mold release agent in the toner is sufficiently exuded on the surface of the toner, therefore, the mold release agent contained in the toner can not sufficiently function, and thus not only non-visual offset easily occurs, but also occurrence of the offset at a stage of less sheet processing is resulted.
The toners disclosed in the aforementioned references are obtained by a pulverization method, and the mold release agent takes an orientation in the produced toner by passing through a kneading. When the mold release agent takes an orientation in the toner, the mold release agent component is exuded onto the toner surface according to an orientation direction in the fixing. Therefore, isotropic exudation to all directions of toners is not performed and thus it becomes more difficult to exert an effect of the mold release agent between the fixing roller and the transfer paper. This way, it is an actual situation that those capable of bearing with the use in the high speed machine can not be obtained using the pulverization toner.
Whereas, in terms of making high-quality picture, it has been carried forward to make particle sizes of the toner small, and improvement of compound for mold release agents and technology to increase an addition amount to the toner are studied as shown in JP-Hei 8-41468A. However, when a large amount of the mold release agent is added, there has been problematic in that the mold release agent is liberated from toner particles and the free mold release agent deteriorates a developer, reduces toner fluidity and causes photoconductor filming.
In JP-Hei 5-88409A, polymerization toner where a toner particle structure is specified, having spherical large islands of the mold release agent in a binding resin is disclosed, but those where no island of the mold release agent is present in the toner particles are found because variation of the addition amount of mold release agent to an individual particle is not dissolved. Also, since the shape of the mold release agent in the toner particle is spherical, contact areas between the mold release agent and the resin becomes small, and thus toner deterioration occurs where charging fault of the toner is resulted from detachment of the mold release agent from the toner particle.
Moreover, those in the above references have the structure where one to two islands of the mold release agent with large diameter are present around a middle of the toner particle, and thus, it takes long to make the mold release agent exude onto the toner surface in the fixing. Thus, there is also a problem that a sufficient layer of the mold release agent can not be formed between the fixing roller and the toner. It has not been accomplished that the toner capable of bearing with the use for the high speed machine is brought to market by the conventional technology.